This invention relates to optical disk storage units, and more particularly, to runout compensation for the tracking servo system in optical disk storage units.
Optical data storage units which utilize a disk to optically store information typically employ a servo system which controls the positioning of the read/write head to provide direct access to a given track of data. See, for example, U.S. application Ser. No. 438,133, filed Nov. 1, 1982, Stahl et al, now abandoned, its continuing application Ser. No. 755,953, filed July 16, 1985, now abandoned, its continuing application Ser. No. 917,259, filed Oct. 8, 1986 and corresponding PCT application, International Publication No. WO84/01849, published May 10, 1984, which show an exemplary servo system for an optical disk storage unit.
Two tracking servo systems are most often employed to enable accurate reading and writing of data from or to data tracks. A coarse servo tracking system radially positions the optical head over a desired data band on the disk. A fine servo tracking system then optically (through the use of tracking mirrors attached to a galvonometer) positions the read/write beams on a desired track(s) within the data band.
Galvonometer controlled tracking mirrors have a limited radial range, hence the necessity for a coarse servo system that: (1) radially positions the optical head over a desired data band where the desired data track is located (coarse seeking), and (2) maintains the head in this desired position (coarse tracking). The coarse tracking servo accurately maintains the head with respect to a precision coarse servo track previously written on the disk, which coarse servo track is adjacent to the data band of concern.
Because replaceable media, or disks, are used, the disks cannot be perfectly aligned on the spindle of the drive. Runout caused by the eccentricity of the tracks and bands as a function of the rotation of the disk surface is associated with any given coarse servo track and its respective data band.
The coarse tracking servo must be able to sense this runout and eliminate its effects so that as far as the fine tracking servo is concerned, it is as though the disk were always "perfectly aligned" on the spindle. Otherwise, the limited range and tracking capability of the fine tracking servo will be severely challenged, and in some instances it may be unable to properly follow the desired track.
In an exemplary embodiment, runout may easily exceed 100 .mu.m. (microns). The coarse tracking servo is designed to reduce this runout, as seen by the fine servo system, to around 5 .mu.m. Unfortunately, when the gain of the coarse tracking servo is increased sufficiently to reduce the effective runout to around 5 .mu.m., there may exist undesirable coupling between the fine and coarse tracking servo that adversely affects the overall performance of the optical storage system.
It is an object of the present invention to provide an improved coarse/fine tracking servo system wherein the undesirable coupling between the coarse and fine tracking system is minimized.
It is a further object of the present invention to provide a coarse tracking servo system that includes a runout compensation feature, wherein undesirable runout may be measured and compensation may be made therefor, thereby allowing the coarse trackng servo system to eliminate the undesirable runout without the need of increasing the gain of the coarse tracking servo to the point where the coarse tracking servo is adversely coupled to a fine tracking servo used in conjunction therewith.